Ganassi duo Scott Dixon and Dario Franchitti ran first and second in the 100th anniversary Indianapolis 500 approaching half-distance.
Dixon led the majority of the race to that point, with Franchitti making steady progress from his ninth place on the grid and briefly getting into first place before his teammate repassed.
Polesitter Alex Tagliani has stayed right with the Ganassi pair for Sam Schmidt Motorsports, with the affiliated Bryan Herta Autosport car of Dan Wheldon the other main contender for victory so far, exchanging third place with Tagliani during the last stint.
The Penske and Andretti Autosport teams are struggling to make an impact, with no cars in the top 10 heading towards half-distance.
Championship leader Will Power is running near the tail of the field after losing his left rear wheel as he left his first pitstop, forcing him to complete a lap on three wheels under yellow.